My present invention relates generally to thermal power plants. More particularly, the invention relates to a novel dry cooling module and system for dissipating the waste heat of a steam-electric generating power plant.
A modern steam-electric generating power plant produces steam by heating feedwater in a boiler with the heat generated from consuming fossil or nuclear fuel. The steam is used to drive a turbine which is mechanically coupled to an electric generator and the exhaust steam from the turbine is directed into a condenser. The condensate from the condenser is collected and returned as feedwater to the boiler. A suitable coolant is, of course, circulated through the condenser to cool and condense the steam therein. This coolant ordinarily is either a liquid or a gas. The liquid coolant used is normally water and the gaseous coolant used is usually air.
The modern steam-electric generating power plant has a thermal efficiency of about 40% and most of the remaining heat must be disposed of as waste. For a 1000-megawatt (MW) fossil fuel power plant, about 45% of the input heat energy is discharged through the condenser coolant and about 15% is lost up the smokestack and in the ash. The condenser commonly used to condense the exhaust steam from the turbine employs water as the coolant which is circulated through the condenser. Condenser cooling water flows of over 400,000 gallons per minute (gpm) are necessary for 1000 MW power plants, for example. While air is a perfectly good coolant, condensers employing air as the coolant to condense the exhaust steam from the turbine would require very large areas of heat exchange surfaces and huge volumes of air circulated thereover for the 1000 MW power plants.
In many of these power plants, the condenser cooling water is obtained from one point of a river, lake or sea and circulated once through the condenser. This heated water is then returned to its source at another nearby point thereof. The heat load deposited by the returned water in its source may create potential thermal effects, however, and such once-through cooling is becoming less acceptable environmentally. To avoid any thermal effects, large cooling ponds are often utilized or cooling canals can be used wherein the heated water from the condenser enters at one end of a canal, cools naturally as the water traverses the canal's length, and exits suitably cooled at the other end into a river, lake or sea.
Still, there are other problems involved with once-through cooling. It is, for example, more difficult to control the quality of the cooling water received from a river, lake or sea because of its variable concentration of salts and other impurities. Also, and of increasing importance, the rapid growth in demand for more power everywhere is continually diminishing the adequacy of available cooling water supplies.
In order to reduce the quantity of cooling water supply needed for eliminating the waste heat of a large power plant, both air and water can be used as coolants. In this instance, the cooling water heated following circulation through the condenser of the power plant can be cooled by evaporative cooling or dry cooling in a "wet" cooling tower or "dry" cooling tower, respectively, and the cooled water recirculated to the condenser to repeat the cycle. Air from the atmosphere is the coolant circulated once through either the wet or dry cooling towers.
In the wet cooling tower, the cooling water heated from circulation through the condenser is caused to fall through a draft of air and most of the heat is dissipated to the atmosphere by evaporation of a small portion of the cooling water. The rest of the water is collected at the bottom of the tower and returned to the condenser for recycling. In the dry cooling tower, the heated cooling water from the condenser passes through heat exchange cooling coils of the tower and a draft of atmospheric air is circulated exteriorly of the cooling coils. The cooled water is collected from the coils at the bottom of the tower and returned to the condenser for recycling.
The wet cooling tower is, of course, more effective and efficient than the dry cooling tower. However, there are greater losses of the circulating water with the wet cooling tower due to blowdown (process of bleeding off part of the water to remove dissolved salts or other impurities which might interfere with system operation), drift (water loss from the tower as fine liquid droplets carried off by the air coolant), evaporation, and leakage. There are, for example, roughly 0.3% (of the water circulated) in blowdown loss, 0.2% in drift loss and 1% in evaporation loss for each 10.degree. F. of cooling accomplished. Makeup water required is of the order of 12,000 gpm for the 1000 MW power plants with cooling water flows of over 400,000 gpm to provide a tower cooling range of about 20.degree. or 30.degree. F.
In addition, the wet cooling tower can cause undesirable fogging and icing conditions at certain times and which conditions often turn out to be quite hazardous. These conditions are avoided with the dry cooling tower which is also easier to maintain than the wet cooling tower. As mentioned previously, however, very large areas of heat exchange surfaces are required in the dry cooling tower and the cooling coils providing such surfaces can make the conventional dry cooling tower over twice as expensive as a comparable wet cooling tower.